Introduction

Phytophthora blight, caused by the oomycete Phytophthora capsici, has become one of the most serious threats to production of cucurbits and peppers, both in the United States and worldwide (2,6,8,9,11,17,18,22,27,30). Phytophthora capsici was first described by Leonian on pepper in New Mexico in 1922 (20). In 1931, Tucker (38) classified it as a species of the genus Phytophthora and considered P. capsici as a host-specific fungus pathogenic on pepper. Subsequently, taxonomists (23,36,37) studied Phytophthora isolates from various hosts in the world and re-described the taxonomy of P. capsici.

Recently, the incidence of Phytophthora blight on cucurbits has dramatically increased in Illinois (2,32) and other cucurbit-growing areas in the world (8,9,17,18), causing up to 100% yield loss. Cucurbit industries, particularly processing industries, are seriously threatened by heavy crop loss resulting from Phytophthora blight. For example, outbreaks of Phytophthora blight have threatened the processing pumpkin and other cucurbit industries in Illinois, where 90% of processing pumpkins produced in the US are grown (2,3) (Figs. 1 and 2). Similarly, the pickling cucumber industry of Michigan is jeopardized by the increased occurrence of Phytophthora blight (8). Because of heavy crop losses the growers often have to abandon their own farms for cucurbit production and move into different areas, sometimes traveling more than 50 miles, to find fields not infested with P. capsici.

Fig. 1. A processing pumpkin field (and pumpkins, inset) at harvest in Illinois.

Fig. 2. A cucurbit fruit showcase at a farm in Illinois.

At present, no single method provides adequate control of P. capsici on cucurbits. No cucurbit cultivar with measurable resistance is available. In addition, crop rotation is virtually ineffective because the pathogen can survive for several years in the soil (8,40), and it can infect more than 50 plant species, including several weed species (6,32). Research is clearly needed to develop effective strategies for the management of Phytophthora blight caused by P. capsici on cucurbits and other vegetables.

Geographical Distribution

Phytophthora capsici on cucurbits was first reported in 1937 in Colorado and California (15,33). Since then Phytophthora blight has been observed in cucurbit growing areas throughout the world (4,5,6,7,40). Phytophthora capsici infection commonly occurs in temperate, subtropical, and tropical environments.

Symptoms and Signs

Phytophthora capsici can strike cucurbit plants at any stage of growth. The infection usually appears first in low areas of the fields where the soil remains wet for longer periods of time (Fig. 3A). The pathogen infects seedlings, vines, leaves, and fruit (Fig. 4).

Fig. 3. Post-emergence damping-off of pumpkin seedlings: (A) a low area in a pumpkin field with severe seedling death; (B) damping-off of a seedling.

Damping-off.Phytophthora capsici causes pre- and post-emergence damping-off in cucurbits in wet and warm (20 to 30°C) soil conditions (5,6,12,40) (Fig. 3). In seedlings, a watery rot develops in the hypocotyls at or near the soil line, resulting in plant death (Fig. 3B). Post-emergence seedling death is preceded by plant wilting. Mature plants show symptoms of crown rot (Fig. 4B). Initial symptoms include a sudden, permanent wilt of infected plants without a change in color (40) (Fig. 5). The wilt of leaves progresses from the base to the extremities of the vines. Often plants die within a few days of the first appearance of symptoms or after the soil is saturated by excessive rain or irrigation. The stem near the soil line turns light to dark brown and becomes soft and water-soaked. Infected stems collapse and die. Tap and lateral roots of infected processing pumpkin plants usually do not exhibit any symptoms. Following death of the foliage, roots may give rise to new vines if environmental conditions become less conducive for disease development. Phytophthora damping-off may result in partial to total loss of the crop.

Fig. 5. Wilt of a pumpkin plant as a result of crown infection with P. capsici.

Vine blight. Vines can be affected at any time during the growing season (2,6,40). Water-soaked lesions develop on vines (Fig. 6). The lesions are dark olive in the beginning (Fig. 6A) and become dark brown in a few days (Fig. 6B). The lesions girdle the stem, resulting in rapid collapse and death of foliage above the lesion site (Figs. 6C, 6D, and 7). Unaffected parts of the vine continue to grow if no girdling lesion develops along the vine.

Fig. 6. Vine symptoms of Phytophthora blight on pumpkin: (A) lesions on a newly infected vine; (B) a fully-developed lesion; (C) a girdling lesion affecting a part of a vine; (D) crown infection.

Fig. 7. Death of a muskmelon plant as a result of crown infection with P. capsici.

Leaf symptoms. Phytophthora capsici can infect both the petiole and leaf blade (2,40). Dark brown, water-soaked lesions develop on petioles (similar to lesions on vines), resulting in a rapid collapse and death of leaves. Infection of the leaf blade results in development of leaf spots ranging from 5 mm to more than 5 cm in diameter (Fig. 8). Infected areas are chlorotic at first and then become necrotic with chlorotic to olive-green borders in a few days. Under wet and warm conditions, leaf spots expand rapidly, coalesce, and may cover the entire leaf. Under dry conditions, expansion of leaf spots may cease.

Fruit rot. Fruit rot can occur from the time of fruit set until harvest (2,17,40) (Figs. 9 and 10). Fruit rot generally starts on the side of the fruit that is in contact with the soil (Fig. 9B). However, when an infected leaf or vine comes in contact with a fruit, fruit rot will start at the point of contact (Fig. 9C). Also, symptoms on the upper surface of the fruit develop following rain or overhead irrigation, which provides splashing water for pathogen dispersal. Fruit rot also can develop after harvest, during transit or in storage. Fruit rot typically begins as a water-soaked lesion (Fig. 9A), which expands, eventually covering the fruit with white mold (2,17) (Figs. 9 and 10). The pathogen produces numerous sporangia on infected fruit (Figs. 9 and 10). Fruit infection progresses rapidly, resulting in complete collapse of the fruit (Figs. 9D, 10C, and 10D). Phytophthora foliar blight and fruit rot may result in total loss of the crop (2).

Fig. 9. Fruit rot of processing pumpkin caused by P. capsici: (A) a lesion on a fruit; (B) fruit rot developed on the side contacting the soil; (C) fruit rot as a result of falling an infected leaf on fruit; (D) severely infected and collapsed fruit.

Pathogen Identification

Identification of Phytophthora capsici is mainly based on the morphology of sporangia (6). Sporangia of P. capsici are variable in shape and are papillate with long pedicels (Fig. 11). Sporangial shapes are influenced by light and other cultural conditions, and are subspherical, ovoid, obovoid, ellipsoid, fusiform, and pyriform. Sporangia are tapered at the base and are caducous with long pedicels. Length × width of sporangia varies from 32.8 to 65.8 × 17.4 to 38.7 µm (6). Length/breath ratios of sporangia range from 1.3:1 to 2.1:1. Pedicel lengths are highly variable among the isolates, ranging from 35 to 138 µm.

Other characteristics of P. capsici isolates from cucurbits are: (i) colonies grow at temperatures from 10 to 36°C, with optimum temperatures of 24 to 33°C: (ii) growth patterns of colonies are cottony, petaloid, rosaceous, and stellate (Fig. 12); (iii) oospore diameters range from 22 to 34 µm (Fig. 13); and antheridia are amphigynous (Fig. 14). Phytophthora capsici is heterothallic, and both A1 and A2 types may exist in the same field.

Pathogenic and Genetic Diversity of P. capsici

Cucurbit isolates of P. capsici have been reported to be pathogenic on cucurbits, pepper, and tomato (12,15). Polach and Wenster (25) reported distinct pathogenic strains among isolates of P. capsici from tomato, pepper and squash. In North Carolina, Ristaino (26) evaluated the relative virulence of isolates of P. capsici from cucumber and squash on pepper and found differences in virulence among the isolates. In Italy, Tamietti and Valentino (30) grouped P. capsici isolates into 13 classes depending on their ability to infect different plant species (pepper, tomato, eggplant, melon, squash, pea, and French bean). In South Korea, Lee, et al. (18) studied aggressiveness of P. capsici isolates from pepper and pumpkin on pumpkin cultivars and reported significant pathogen-host interactions. In Illinois, Tian and Babadoost (31) reported significant differences among isolates of P. capsici from different locations.

Genetic variation among P. capsici isolates has been reported in vegetable growing areas in the world (10,16,19,31). Different methods have been used to study the genetic variation of fungi (16, 21,28,34,35,39). Internal transcribed spacer (ITS) regions have been used to determine genetic differences among species of Phytophthora (28). Inter-simple sequence repeats (ISSR) amplification is a new technique that could rapidly differentiate closely related individuals within a fungal species (31,39). Amplified fragment-length polymorphism (AFLP) is a recently developed polymerase chain reaction (PCR) technique that provides genetic markers for fingerprinting, mapping, and studying genetic relationships among populations within fungal species (1). Tian and Babadoost (31) used ISSR and AFLP tests to determine genetic differences among isolates of P. capsici from Illinois (Fig. 15). They reported that cluster analysis separated the isolates into four distinct groups, representing four different locations from which they had been collected.

Disease Cycle and Epidemiology

Phytophthora capsici is a soilborne pathogen and survives between crops as oospores in soil or mycelium in plant debris (6,8,24,40). An oospore is a thick-walled sexual spore (Figs. 13 and 14) and is formed when mycelia of two opposite mating types (A1 and A2) grow together. Oospores are resistant to desiccation, cold temperatures, and other extreme environmental conditions, and can survive in the soil, in the absence of a host plant, for many years (8,40). Oospores germinate and produce sporangia and zoospores (asexual spores) (Fig. 11). Zoospores are released in water (Fig. 11B) and are dispersed by irrigation or surface water. Zoospores are able to swim for several hours and infect plant tissues. Abundant sporangia are produced on infected tissues, particularly on affected fruit (Figs. 9 and 10), and dispersed by water or through the air. Sporangia can either germinate and infect host tissues directly, or they can release zoospores, which can then infect the plant. If the environmental conditions are conducive, the disease develops very rapidly.

Soil moisture conditions are important for disease development (8,40). Sporangia form when the soil is at field capacity and they release zoospores when soil is saturated. The disease is usually associated with heavy rainfall, excessive-irrigation, or poorly drained soil. Frequent irrigation increases the incidence of the disease.

Disease Management

No single method currently available provides adequate control of Phytophthora blight. A combination of measures should be practiced to reduce the damage caused by P. capsici on cucurbits (2,8,12,13,14,40). The most effective practice in controlling P. capsici is preventing the pathogen from being moved into a new field. The following practices can help to manage Phytophthora blight in cucurbit fields:

1. Select fields with no history of Phytophthora blight.

2. Select fields that did not have cucurbit, eggplant, pepper, or tomato for at least 3 years. No rotation period has been established for effective management of Phytophthora blight of cucurbits.

3. Select fields that are well isolated from fields infested with P. capsici.

4. Select well-drained fields, or do not plant the crop in the areas of the field which do not drain well.

Conclusion

Phytophthora blight, caused by P. capsici, is and will continue to be a serious threat to cucurbit production in the US and worldwide. There are no cucurbit cultivars with measurable resistance to Phytophthora blight, the pathogen survives in soils for several years, and limited chemical control of the disease is available. New strategies for management of Phytophthora blight are essential. Along with cultural management strategies, research is needed to assess the possibilities of using induced resistance in plants, genetically modified cultivars, biocontrol agents, and eradicant fungicides for control of P. capsici in cucurbits and other crops.